The invention relates to a process for the separation of butanes and butenes by extractive distillation.
In the treatment of the C.sub.4 fraction resulting from steam or catalytic cracking, the separation of the butenes and butanes normally takes place downstream of the extractive distillation of the 1,3-butadiene and the methyl tert. butyl ether (MTBE) synthesis unit. Thus, a C.sub.4 fraction is obtained, which is free from 1,3-butadiene and highly depleted in isobutene.
As butenes are valuable products, it is necessary to separate them from the butanes. Thus, the butenes can themselves be separated into 1-butene and 2-butenes, or can be isomerized into isobutene, which is recycled in the MTBE synthesis unit. The butenes can also be dimerized by the Dimersol (registered trademark) process into products usable in gasoline. This dimerization is a process performed by homogeneous catalysis, in which the catalyst is lost. As the efficiency of the catalyst is proportional to its concentration in the charge, a paraffin-free charge leads to considerable catalyst economies. Moreover, the butenes which have not reacted can be recycled if the butanes have previously been removed from the charge.
It is known to separate the butanes and butenes in a C.sub.4 fraction by extractive distillation. A process has already been described (S. Ogura, T. Onda, Advances in C.sub.4 Hydrocarbon Processing AlChE 1987 Summer National Meeting, Aug. 16-19, 1987) consisting of performing the extractive distillation under pressure with dimethyl formamide (DMF) as the solvent, solvent recovery being performed under atmospheric pressure and the purification of the butenes taking place under pressure. The bottom product of the extractive distillation column is fed into a column under atmospheric pressure, where the butenes are desorbed and leave at the column head with part of the solvent, whilst the bottom product is constituted by substantially pure DMF. The head butenes are then supplied for purification purposes to a pressure column. The solvent recovered at the bottom of the latter column and which still contains butenes returns to the atmospheric column.
The disadvantage of working according to this process is that most of the butenes of the charge have to be distilled twice which, bearing in mind the necessary recompression of the vapors of the butenes, leads to a high energy consumption.